Far-end crosstalk (FEXT) is a major problem significantly limiting the performance of DSL systems. An ITU-T standard (Telecommunication Standardization Sector of the International Telecommunication Union), G.993.5 [1], for cancelling FEXT by means of signal processing, has been developed. This crosstalk cancellation technology is usually referred to as “vectoring” or “DSM (Dynamic Spectrum Management) level 3” technology.
Vectoring technology is assumed to be the core technology of the next generation of DSL for cancelling the FEXT between DSL lines, and thus maximize the DSL system performance. Vectoring technology will play a very important role in FTTx (Fiber To The Node/Cabinet/Curb/Building/Home/Premises, etc.) business, because it enables offering 100 Mbps per user with DSL lines in the last hundred meters, i.e. the distance between the end of a fiber network and the CPEs (Customer premises Equipments).
A schematic downstream vectoring arrangement is illustrated in FIG. 1. The downstream vectoring arrangement shown in FIG. 1 comprises a precoder 102, for pre-cancelling of crosstalk. The precoder is located at the DSLAM (Digital Subscriber Line Access Multiplexer) side 106 of a DSL line bundle or cable 104. The cancellation of FEXT is done at the DSLAM side 106 of the DSL lines 110. Downstream FEXT is pre-cancelled by use of a precoder 102 in the DSLAM, while upstream FEXT is cancelled by use of an upstream crosstalk canceller in the DSLAM (not shown). According to an ITU-T recommendation, a way is provided to estimate the FEXT channel in both downstream and upstream, and to utilize the estimated channel to cancel the crosstalk
To explain the vectoring principle, referring to FIG. 1 and without considering the background noise, the received signals y1, y2, y3 . . . yn at the different CPEs 1-N can be expressed in matrix form as:y=HPx   (1)where y=[y1 y2 . . . yN]T and yi is the received signal at CPE i,X=[x1 x2 . . . xN]T and xi is the transmitted signal of line i, H is the channel matrix, P is the precoding matrix doing crosstalk pre-cancellation, and XT denotes the transpose of the vector X.
Applying a simple zero-forcing technique and setting:P=H−1   (2)results in:y=x   (3)Thus, the received signal equals the transmitted signal, and thus no crosstalk is present in the received signal at the CPEs. Similarly, the upstream crosstalk can be cancelled by post-processing in an upstream crosstalk canceller at the DSLAM side.Partial Vectoring
The vectoring technology is a very attractive solution for VDSL2 [2] cabinet deployment, where vectoring enabled VDSL2 DSLAMs are installed in cabinets where hundreds of lines, typically, are connected to the users. However, fully cancelling hundreds of lines is too costly in terms of signal processing. Therefore, partial vectoring is considered as a practical solution for vectoring by only cancelling a part of the crosstalk/ers to each line, preferably the “strongest” crosstalk/ers. FIG. 2 and FIG. 3 show a simplified partial-vectoring system model for downstream and upstream, respectively. As shown in the system model, the partial vectoring system illustrated in FIGS. 2 and 3 is capable of cancelling a selected subset of the crosstalkers for each line. It has been shown that close-to-optimal performance can be achieved by using partial vectoring. When using partial vectoring, weak crosstalk/ers is/are left unprocessed, and therefore, the use of partial vectoring enables a significant reduction of the computational complexity and cost of vectoring systems.
However, there is a management issue to solve in partial vectoring, namely how to distribute the vectoring resources among the DSL lines, and how to determine which crosstalk/ers that should be cancelled on each line. To manage the partial vectoring capability, ITU-T G.993.5 [1] defines two new configuration parameters related to partial vectoring:                Target Data Rates: referring to the expected data rates, for downstream and upstream, respectively, which are achievable for a line when all lines in the vectored group are active.        Line priorities (LOW/HIGH): partial vectoring should initially allocate sufficient resources in such a way that the target data rate is met for all the lines in the vectoring group. Then, the remaining resources will be distributed to the lines with line priority HIGH first to improve their data rates above the target data rates until they reach the maximum data rates configured. If the maximum data rate condition is met for all the vectored lines with line priority HIGH, the remaining resources are allocated to vectored lines with line priority LOW to improve their data rates above the target data rate.        
ITU-T G.993.5 [1] defines a vectoring initialization procedure, which enables vectoring. This procedure is illustrated in FIG. 4. It should be noted that only the steps related to crosstalk cancellation are shown in FIG. 4, in order to simplify the discussion. Basically, ITU-T G.993.5 defines a joining procedure in which the existing vectored lines, which are already in showtime, are not interfered by the joining lines, which initialize to enter showtime, and eventually the mutual crosstalk between lines are cancelled after certain steps. This defined procedure is very straight forward to apply for full vectoring. However, when applying partial vectoring, it is not clear how to support the requirements of target data rates and line priorities, e.g. during initialization.
Further, when regarding the crosstalk to a specific line, it is not clear how to allocate vectoring resources in order to achieve the best possible result from the allocated vectoring resources. A line i may be subjected to crosstalk from a number of different other lines in the same vectoring group. The crosstalk from all the tones S of another line I to line i may vary over all the tones S of line i. Thus, it is a multi-dimensional problem to determine, and eventually cancel, the crosstalk from each tone of each other line to each tone of line i. It has not even been defined how to determine which crosstalk that is the “strongest” crosstalk to a line, In addition, lines may have different target bit rates and priorities, which should be regarded. All this taken together imply that the task of allocating partial-vectoring resources among lines and within lines is a problem which needs to be solved.
Since it is believed that partial vectoring is of great importance for field deployment for computational complexity reasons, there is a need to have an initialization method, which supports partial vectoring, configured target data rates and line priorities.